Hydrostatic transmission control system



11, 19 69 J, CRYDER ET AL 3,477,225

HYDROSTATIC TRANSMISSION CONTROL SYSTEM Filed June 14. 1961' v 1oSheets-Sheet 1 Nov. 11, 1969 J. R. CRYDER ET HYDROSTATIC TRANSMISSIONCONTROL SYSTEM 10 Sheets-Sheet 2 Filed June 14, 1967 mma A A MVQ 3 mw a1 l 3m Q wm 4w A? ww W H mm mm Q mj 568 F5 Q95 G O m lmn Nov. 11, 1969'I J. R. CRYDER ET AL 3,477,225

'HYDROSTATIC TRANSMISSION CONTROL SYSTEM Filed June 14, 196'? 10Sheets-Sheet 5 IDLE 4.0 )8

J. R. CRY DER ET HYDROSTATIC TRANSMISSFON CONTROL SYSTEM Nov. 11, 196910 Sheets-Sheet Filed June 14, 1967 mmw t mm m Mm NM m my Nov 11, 1969 RCRYDER ET AL 3,477,225

HYDROSTATIC TRANSMISSION CONTROL SYSTEM Filed June 1967 10 Sheets-Sheet6 ENGINE H (I93 92 194 i IIIIIIE- um i 1 v. 1 I65 I 9Q: a Z [3 2 2 E 35aj i 31 30L I 30 .55 32 I 3 I I00 31 14 0 o o I :3 6 8 0 NZ. 5.; j 36a 3,

or w Nov. 11, 1969 J. R. CRYDER vs'r AL 3,477,225

HYDROSTATIC TRANSMISSION CONTROL SYSTEM Filed June 14, 196'? 2 1OSheets-Sheet 7 I 2|4 I 2 1 08a. L- -i 33 i Nov. 11, 1969 J. R. CRYDER ETAL HYDROSTATIC TRANSMISSION CONTROL SYSTEM Filed June 14, 196'? 10Sheets-Sheet 8 M E r m m a B 1g u 2. S Q a? A} E mm v ww Q: m Q 0 N2 urim 5 HY s m x u I i 5 [3. mm 1 v my m F m w 11, 1969 D R ET AL3,477,225

HYDROSTATIC TRANSMISSION CONTROL SYSTEM 7 Filed me 14, 19s? 10Sheets-Sheet 9 E J.E

I80- SPOT TURN g [50' \85 /20. (I) REV. TRACK TURN STARTS E 90 YSTEERING STARTS Z a 60- H bJ :3 0 30 SPOOL TRAVEL (IN) Nov. 11, 1969 J.R. CRYDER ET AL 3,477225 HYDROSTATIC TRANSMISSION CONTROL SYSTEM FiledJune 14, 1967 10 Sheets-Sheet 10 'mLWE% United States Patent HABSTRACTon THE DISCLOSURE" Hydrostatic transmission, systems which transmittorque via =fiuid pressure circulating between a pump and motor, are nowsufficiently perfected for use in work vehicles. Infinitely variableinput to output ratios-are available in such transmissions which makethem especially useful-in'tractor type vehicles. However, for highefiiciency the-transmissionratio must be properly and continuouslyadjusted for maximum performance. Such control is accomplished in thisinvention by employing a separate control pump geared to thetransmission power source, a valve controlled differential pressurestack connected to receive the output of the control pump whereincha'nges in its valve'position will effect changes in pressuresdifferential occurring within the stack, pressure responsive meanshaving elements displaceable by pressure differential connected to thestack so that pressure differentials occurring therein are communicatedto the elements and linkages connecting the elements with control meansin the transmission for changing its ratio. Also included is flowsensitive system which is connected acros's'the differential pressurestack to reduce the pressure differential if the output of the controlpump drops due to a decrease in engine speed. The basic control systemcan be employed with a variety of subsystems to provide steering intrack-type vehicles, reversing and the like.

BACKGROUND OF THE INVENTION Advances in the technology of hydraulictranslating devices (hydraulic pumps and motors) now allow hydrostatictransmissions to be employedin vehicles having high drawbar horsepowerrequirements. With its infinitely variable Speed ratio between theengine and the ground speed of the vehicle the hydrostatic transmissionoffers the ability to obtain the maximum drawbar horsepower over thevehicles full speed range. An early hydrostatic transmission for atrack-type vehicle is disclosed in US. Patent No. 2,036,437 issued toRuediger, but at that time the technology of translating units was notperfected sufficiently to bring it to reality.

With the new translating devices now available, the limited gear stepsin work vehicles, associated clutching, shifting, braking and engineacceleration and deceleration, may become a thing of the past.Translating devices suitable for hydrostatic transmissions in workvehicles are disclosed in copending application Ser. No. 564,875entitled Hydrostatic Apparatus wherein many of the advantages ofhydrostatic transmissions are noted.

For a hydrostatic transmission to have a broad speed range without largepump and motor units, it is necessary that the displacement of both itspump and motor be varied in displacement. This avoids the need for bothhigh and low speed motors, such as disclosed in US.

3,477,225 Patented Nov. 11, 1969 "ice Patent 2,541,290 issued toRobinson, or unduly large translating units to obtain the desirablespeed ranges.

Hydrostatic transmissions in which bothithe pump and motor are variabledisplacementiinits present a special problem in control since thechanges in displacement of the pump and motor relative to one anothershould be properly sequenced for efficient operation and to providedesirable torque ratio characteristics in the transmission. In aproperly sequenced transmission, the pump will have zero displacement ata zero speed condition, and the motor will be at its maximumdisplacement. For acceleration of the vehicle, the pump is increased indisplacement toward a maximum displacement, while the motor remains atits maximum displacement so it will develop maximum torque at minimumpressures for accelerating the vehicle.

After the pump reaches its maximum displamement, the vehicle is atmaximum speed unless the displacement of the motor is variable. Theneven with small size translating units, a broad speed range is obtainedby decreasing the displacement of the motor to further increase thespeed of the vehicle with such a transmission.

Also, these transmissions in a vehicle should be properly sequenced froma high speed to a lower speed, somewhat similar to downshifting aconventional transmission. Thus, for a speed decrease from maximumtransmission speed, the motor displacement is first increased to itsmaximum to initially slow the vehicle and thereafter the pump is movedtoward its zero displacement to further retard vehicle speed.

If proper sequencing is not employed desirable torque ratiocharacteristics are lost with an accompanying decrease in transmissionefficiency; operation of the transmission may become rough in somemodes; retarding (braking) action through the transmission is reducedduring deceleration; and unnecessary high pressures will be developed inthe transmission.

Some control systems for hydrostatic transmissions have been designedwhich sense the pressure in the fluid loop connecting the pump and motorand use this pressure to control the transmission. However, those typesof control systems are not satisfactory since, in many cases, the enginedriving the transmission ,must also power mounted or towed implementswhich make power demands on the engine, without changing the pressure inthe hydrostatic loop. Further, loop pressure can vary independent ofengine load. g

.Accordingly, it is a purpose of this invention tosprovide a simple butreliable control system-,for hydrostatic transmissions which is capableofsensing the total load on an engine and automatically...adjusting ahydrostatic transmission for maximum output in relation to thepoweravailable. r.

Another purpose of this invention is,t0 provide a control system forpaired hydrostatic transmissionsin tracktype vehicles whereinthetransmissions can-be independently controlled for steering the vehicles,

Also, it is a purpose of the invention to provide complementarycomponents for the basic control system which increase its flexibilityand improve its performance.

SU MMA-RY Briefly, the above purposes and advantages as well as manyothers, can be accomplished in a basic control system for a hydrostatictransmission comprising a positive displacement control pump geared tothe prime mover driving the transmission, a venturi means connected tothe pump so the output of the pump passes therethrough, a differentialpressure control stack also connected so the output of said control pumppasses therethrough, a manually adjusted valve in said differentialpressure stack for controlling the differential pressure in the stack,biased valve means associated with the venturi means and connectedacross said differential pressure stack operable to reduce thedilferential pressure in the stack when the engine speed decreases underload, pressure reciprocated pilot means connected to said stack andhaving a movable element therein which changes position in response tothe differential pressure in the stack, and linking means connectingsaid element to the servo-units in the transmission for controlligdisplacemet of translating units, and thus, transmission ratio.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 diagrammatically illustratesthe basic control system associated with a hydrostatic transmission;

FIG. 2 illustrates a known relief and replenishing group plus a highpressure relief system for the hydrostatic transmission shown in FIG. 1;

FIG. 3 diagrammatically illustrates a modification to the basic controlsystem shown in FIG. 1 which provides variable engine speed compensatingmeans for the control system and linear servo systems for transmissionratio control, along with sequence system valves for forwardreverseshift;

FIG. 4 is a block diagram showing the relationship between FIGS. 5 and6;

FIG. 5 shows part of the control system when employed for dualhydrostatic transmission systems in tracktype vehicles with provisionsfor steering;

FIG. 6 discloses the complementary parts of the control system shown inFIG. 5 and also the transmission units;

' FIG. 7 shows the dual transmissions shown in FIG. 6 mounted in atrack-type vehicle;

FIG. 8 illustrates a relief and replenishing valve unit which isemployed with the integrated control system shown in FIGS. 5 and 6,providing additional control features in the system;

FIG. 9 illustrates a safety valve system which cooperates with therelief and replenishing valve shown in FIG. 8 to deactivate thetransmission for operator safety;

FIG. 10 diagrammatically illustrates a steering control system employedwith track-type vehicles using dual transmissions as shown in FIGS. 5and 6;

FIG. 11 shows schematically a hydraulic circuit for the steering controlsystem of FIG. 10;

FIG. 12 graphically shows the proportion between control movement andresponse of the steering circuit; and

FIG. 13 illustrates a variable orifice flow device which can be used inplace of simple ball checks and parallel orifice units in moresophisticated control systems for smooth transmission response.

DESCRIPTION OF EMBODIMENTS In understanding the control system it ishelpful to associate it with a hydrostatic transmission. Therefore, FIG.1 illusrtates the control system with a hydrostatic transmission havinga variable displacement pump 31 and motor 32 connected together in ahydraulic loop circuit composed of an upper conduit 33 and a lowerconduit 34. Upper and lower with reference to the conduits are merelyfor convenience and not indicative of position or structure.

The transmission illustrated in the drawings uses variable displacementtranslating devices which are illustrated in copending application Ser.No. 564,875 entitled Hydrostatic Apparatus having a common assignee.These particular translating units are suitable for highpressureoperation since the hydrostatic loop is completely rigid with theconduits 33 and '34 joined directly to rigid trunnions which extendcompletely through these units. This forms a very rigid transmissionstruction.

'For actuation of the transmission the units are swung about theirtrunnion to change the displacement. Pump actuator 35 through its rod35a swings the pump for changes in displacement with motor actuator 36swinging the motor about its trunnion via rod 36a. The pump actuator iscontrolled by the pump rotary servo unit 37, while the motor actuator isindependently controlled by a separate motor rotary servo 38. Theseservo units are mounted adjacent to the pump or motor which they arecontrolling so that follow-up linkages 37a and 38a can be connected tothem for controlling position correspondence in the servo units.Hydraulic lines 39 connect the respective actuators with theirassociated rotary servo.

The rotary servo valve units can be of a conventional type, such asshown and described in the U.S. Patent No. 1,773,794 issued toSchneider, and control the flow of pressurized fluid from line 40 totheir associated actuator in order to change the pump or motordisplacement. A pump servo control arm 41 positions the pump servo,which in turn allows fluid to flow toactuator 35 until the pump positionhas swung sufficiently that follow-up bracket 37a repositions parts ofthe servo to stop the flow of fluid when position correspondence isachieved. During movement of the actuator fluid is drained from theunpressurized end of the acuator to reservoir via the servo unit anddrain 43.

Motor rotary servo unit 38 operates in the same manner, thus movingmotor servo control arm 44 will change displacement of the motor byallowing pressurized fluid to flow from line 40 to the actuator 36 vialine 39 until such time as follow-up bracket 38a has repositioned theservo unit to stop fluid flow at position correspondence. Fluid from theunpressurized side of the motor actuator is drained to reservoir via theservo and drain 46.

It should be appreciated that the particular translating unitsillustrated are not intended to limit the invention, and that othertypes of fluid translating units, such as those shown in U.S. Patent3,274,946 issued to E. E. Simmons, could be employed and controlled bythe servo control units associated with the control system.

In most hydrostatic transmissions leakage is not sufficient to allowenough make-up fluid to be added to the fluid loop for cooling purposes.Thus it is necessary to utilize a relief and replenishing group 47(enclosed in broken lines), such as shown in FIG. 2, so some of thefluid can be removed from the loop and make-up fluid added for coolingthe transmission. Operation of such a group is discussed in order that afully integrated control system according to this invention may beunderstood.

In FIG. 2 a prior art relief and replenishing group 47 with ahigh-pressure relief is illustrated. In a hydrostatic transmissioneither the upper or lower conduit 33 or 34 may be operated at a higherpressure, as the working side of the loop, depending upon the power flowthrough the transmission. Thus the relief and replenishing group must beable to remove and add hydraulic fluid from either side of the loop(conduit 33 or conduit 34). Typically this is accomplished by twoseparate sections in the relief and replenishing group, which, forconvenience, can be designated as replenishing section 51 and the fluidrelief section 52 (outlined with broken lines).

In the replenishing section two spring-biased ball checks 53 and 54 arein communication with conduits 33 and 34, respectively, through stubpipes 53a and 54a. These checks, illustrative of type of checks whichcan be employed, are arranged so that pressurized fluid from line 40 canpass through one or the other check valve into the hydraulic loop,providing a positive pressure in the conduit operating at the lowerpressure, preventing pump cavitation and adding cool fluid to the loop.The higher pressure in the conduit operating as the working side of theloop will close its associated ball check so that power will not be lostby flow of pressurized fluid therethrough.

The replenishing fluid from line 40 is precooled in order to providetransmission cooling, and the fluid relief section 52 is used to removefluid from the loop so that a greater amount of cool replenishing fluidcan be added through ball check valves. Hydraulic fluid in the loop' isremoved through'the "relief fluid dump valve 55 subsequent tocirculating th'roughthe motor.'The dump valve is connected betweenthe'condu'its 33 and 34' with short pipes 55a. A spring centered spool56 in thisvalve is arranged so its opposite ends are incommunicationwith the presurized fluid in conduit 33 and 34 via the stub pipes.Normally this spring centeredspool will'remain in a neutral positionuntil either conduit 33 or 34 operates at a'higher pressure, at'whichtime itwill be displaged in a direction to open a port 57 communicatingwith the conduit operating at the lower pressure. As this occurs areduced center portion 5612 will open'a flow passage through one of thetap'conduits 58 on the main loop', through chamber 550', conduit 59,relief valve 60 and cooler 61 to reservoir. Opening this path allowsadditional fluid to be removed from the loop, subsequent to passingthrough the motor so that a greater amountof replenishf ing fluid can beintroduced through one of the ball check valves, as previously noted. vf I j 'Also, 'it'is desirable to incorporate an over-pressure reliefvalve system 62 in the loop of a hydrostatic transmission. In FIG. 2such a system is connected between conduits 33 and 34 with shortconnecting conduits 620. These short conduits communicate with separatechambers which are closed by a spring-loadedpoppet valve 63 whose biasis adjusted to control the maximum pressure in the loop. The chamberbehind each poppet is connected with the short conduit 64 with thechamber, closed by the opposite poppet so that when over-pressure occursdisplacing one of the poppets, fluid can vent from one side of the loopdirectly to the opposite side, bypassing the motor. A ball check 64d inconduit 64 prevents reverse flow in each line. w h

The above description covers a typical hydrostatic transmission and itscomponents'with which the novel control system of this inventionis'employed. From the above discussion, it canbe appreciated that thecontrol system must coopearte with the servo-control units of thepump'and motor in order to achieve proper sequencing of thetransmission. n

Normally such a transmission as described above will be directlydrivenby an engine 65, through shaft as illustrated in FIG. 1. It is notnecessary, though optional, to have a clutch between the engine and thepump since the pump can be positioned for zero displacement 'toneutralize the power flow in the transmission Without disconnecting thepump. The engine also drives theimplement pump 67 through shaft '68,which also makes power demands on the, engine during operation ofvehicle- 'r'nounted or towedimplernents. a

' The controlfsystern of this invention is designed to properlysequence'a hydrostatic transmission, like the one described above, underall conditions of load. It automatically adjusts the transmission forthe maximum power output consistent with power availablefromtheenginew'ithin a manually set speed range.

As previously noted, the control system includes a positive displacementpump geared to the engine 65,.a venturi means connected to receive theflow of the pump, a differential pressure stack also connected toreceive the flow of the 'pump, a manually adjusted valve .in' thedifferential pressure stack for controlling the pressure differential,biased valve means associated with the venturi connected into the stackwhich also controls the differential pressure therein in response tochanges .in engine speed, pressure reciprocated pilot means connectedacross the stack and having an element movable therein which changesposition in response to differential pressure, and linking meansconnecting said-elements to servovalve units for controlling theactuators in the transmissron.

' -M0re specifically, the control pump 70 is connected to the enginethrough its shaft 69 andis of the positive displacement type. Thus, theoutput of the control pump will be directly proportional to the speed ofengine 65. Pressurized fluid from the pump flows via conduit 71 throughventuri 72 and conduit 73 to the differential pressure stack 74. Thedifferential pressure stack can be fabricated in a number of ways, butthe simplest is the use of two parallel tubes 75 and 76, with tube 75connected directly to the output of the venturi and a manual valveleading from it to tube 76. A conduit 77 carries the fluid from tube 76of the stack to reservoir via relief valve 78, or through pres surizedline 40 to the relief and replenishing group 47 of the transmission(previously described) from which it is eventually returned toreservoir.

Pressurized fluid from tube 75 passing to tube 76 of the differentialpressure stack must pass through the manually-operated speed controlvalve 79, or an underspeed valve 80, the latter normally being closedwhen the engine is operating at rated speed. The speed control valveincludes a modulating valve spool 81 which is positioned by themanually-manipulated speed lever 82 through linkages 83. Movement ofthis spool restricts or opens the pas sage between the tubes of thedifferential pressure stack. As the restriction increases the pressurein tube 75 will increase above that of tube 76, since the pump flow vol-7 ume will remain constant at a selected engine speed.

Thus, the greater the restriction of the passage by the spool, thegreater pressure differential there will be between the two tubes of thestack.

The location of the venturi in the control system is not critical solong as all the control pump flow passes therethrough. It can be locatedeither upstream or downstream of the differential stack since it isresponsive to the flow rate from the control pump and cooperates withthe underspeed valve to open a parallel path to that controlled by thespeed control valve when the engine speed drops off and control pumpoutput decreases.

Basically, underspeed valve 80 is designed to function when the enginefalls below its rated speed for which the control system is designedwhich results in a decrease in output from the control pump 70. Whenthis occurs the pressure in the low-pressure tap 72a of the venturi willincrease causing a greater pressure to be reflected in a chamber behindvalve spool 80a containing the spring bias in the underspeed valve.Thus, the pressure from the low-pressure tap of the venturi and thespring bias overcomes the pressure in the inlet throat of the venturivia line 71a reflected on the opposite side of the spool. As long asthis pressure exceeds that in the low-pressure venturi tap and thespring 84, the valve spool will be urged into the spring to .bottom on ashoulder in the valve body to close off this passage between the tubesof the stack through the underspeed valve. Therefore, decreased outputof the control pump due to a reduction in engine rpm. will graduallyopen a flow path between the tubes through the underspeed valve causingthe previously established pressure differential in the stack todecrease in proportion to the drop in engine r.p.m. thereby reducingtransmission torque requirements until the engine can maintain-ratedr.p.m. Normally this underspeed valve is set for actuation when theengine speed drops from 3 to 10%, but can be adjusted for a desiredsensitivity.

From'the above description it should be appreciated that the aboveelements of the control system can create a pressure differential in twotubes of the stack which is reduced atuomatically when an underspeedcondition exists by the opening of the path through the underspeedvalve. By interconnecting pressure responsive means which have elementsthat move in response to changes in the differential pressure, it ispossible to utilize the system to control a hydrostatic transmission.Normally these pressure sensitive elements are connected toservo-control means since they are not generally capable of generatingthe forces necessary to change the ratio of the transmission.

Typical of such pressure sensitive elements is pump pilot cylinder 90which controls servo-control arm 41 of the pump servo 37 by a connectionto the arm through adjustable linkages 91. Its piston element 92 iscentered in the pilot cylinder by springs 93, and the opposite ends ofthe pilot cylinder are in communication with opposite tubes of thedifferential pressure stack 74 so that any pressure differential in thestack will be reflected on opposite sides of the piston element.Obviously, some preload to the centered position is desirable to insureaccurate positioning at zero differential.

Connected in this manner, any pressure differential existing in thestack will cause the piston of the pilot cylinder to be displaced in adirection sufficient to balance the differential against increasingspring bias. It should be appreciated that the pilot cylinder operateson pressure differential and thus absolute pressures in the system arenot critical.

Since the pump unit is the reversing unit of the transmission andreverses the power flow (direction of fluid flow), it is necessary toprovide means for reversing the connection of lines 94 and 95 connectingthe opposite ends of the pump pilot cylinder to the differential stackto enable its pilot cylinder to operate in both forward and reverse.Forward-reverse valve 96 is used for this purpose, and can switch theconnection of line 94 from tube 75 of the differential stack to tube 76of the differential stack for reversing. Line 95 is simultaneouslyswitched from tube 76 to tube 75, which allows the linkage connectingthe piston element to the servo-control arm 41 to properly position thepump for the reverse power flow.

Since the pump pilot cylinder tends to react rapidly to pressure changesin the differential pressure stack, transmission control may be roughunless means are provided to smooth out the rate of movement of thepiston element in the pump pilot cylinder. This is accomplished by aparallel ball check and orifice device 97 between the forward andreverse valve and the tubes of the differential pressure stack, as shownin FIG. 1. Since tubes 75 of the differential pressure stack will alwaysbe operating at the high pressure when the transmission is running theball check in the line connecting this tube to the forward-reverse valveis oriented to meter the flow to the valve. This limits the ratepressure change in the pilot cylinder and thus smooths out movement ofits piston element. A similar device incorporated in the connectionbetween valve 96 and tube 76 of the differential stack is reversed sothat flow is unrestricted by the ball check from this tube toward thepilot cylinder, but is restricted from the cylinder toward tube 76.Through the above arrangement, the piston in the pilot cylinder willreact smoothly for good transmission response.

In the above control system, it should be appreciated that there isessentially no net fluid flow required to operate the system since itresponds to differential pressure and movement of the pilot cylinderwill return the same amount of fluid from one side as is being receivedby the other so that the net flow is always zero. Thus, actuation of thecontrol system does not result in any pressure fluctuation due to fluidflow demands.

As pointed out above, the transmission is reversed by reversing thepower flow (fluid flow) through the pump, and thus the motor 32 willautomatically reverse with the change in direction of fluid flow. Thus,motor pilot cylinder 100 is connected directly to the differentialpressure stack without a forward-reversing value. However, the lines 101and 102 connecting the motor pilot cylinder to the differential pressurestack contain parallel ball check and orifice devices 97 to smooth outthe actuation of the motor pilot cylinder in the same manner asdescribed for the pump pilot cylinder. Other modulating means may beemployed in place of the ball checks and orifice devices.

As previously noted, the proper sequencing of the transmission requiresthat the pump 31 be at a maximum displacement in either direction beforethe displacement of the motor 32 is decreased for increasing the speedof the transmission. This is accomplished in the control system of thisinvention by a difference in construction in the motor pilot cylinder.Basically, the motor pilot cylinder has a single spring 103 which biasesits piston 104 to one end of the cylinder against a stop 104a. The sideof the piston without the spring is in communication through line 101with tube 75 of the differential pressure stack, while the biased sideof the piston is in communication with tube 76 of the differentialpressure stack via line 102. The motor pilot cylinder has an adujstablelinkage 105 connecting its piston to the control arm 44 of the motorservo 'unit 38.

Using the above construction and proper selection of the spring biasingunits in the pump pilot cylinder and motor pilot cylinder, propercontrol of the transmission can be accomplished by selecting the bias inthe motor pilot cylinder so the piston of the pump pilot cylinder willbe displaced a maximum in either a forward or reverse direction at alower pressure differential than is necessary to effect movement of thepiston in the motor pilot cylinder against its large spring bias. Thus,as differential pressure in the stack increases, the pressure reflected.on the piston of the motor pilot through line 101 will eventuallyincrease sufficiently to move piston 104 against the spring bias andthrough the linkage with the servo-control arm effect and adjustment ofthe motor for an increase in speed, after maximum pump displacement.

Through the unique arrangement described above, the transmission willalways be properly sequenced by the control system. For example, as thedifferential pressure in the stack increases from zero, the pump pilotcylinder will be first to react and will be displaced to a maximum priorto the actuation of the motor pilot cylinder which requires a slightlyhigher pressure for it to react. Thus, proper sequencing from low tohigh speed and from high to low speed in the transmission will always beachieved. Also, during all phases of sequencing this novel system, thetransmission is automatically adjusted for the power available since thecontrol pump 70 will always reflect engine speed and any decrease inspeed will automatically change the differential pressure in the stackthrough the action of the underspeed valve until the engine can maintainits rated speed, or a close approximation thereof. Therefore, anyhorsepower demands from the implement pump 67 or other engine-connectedunits will not lug down the engine since the transmission willautomatically reduce vehicle speed until the engine is able to hold itsrated speed. This means that the implement circuits will operate morerapidly (higher pump r.p.m.) and that the maximum efficiency as a unitcan be obtained from the vehicle using this control system with ahydrostatic transmission.

The basic control system shown in FIG. 1 is designed for an enginerunning continuously at its rated speed, and in order to provide thebasic control system with greater flexibility so that an operator canselect various engine speeds below rated, a modified system is shown inFIG. 3. Also, in this design the actuators for controlling thedisplacement of the pump and motor and the servo units are shown as alinear system. Other than these changes the system is very similar tothat shown in FIG. 1 and identical parts are similarly numbered.

In this modified system, to operate the transmission control systembelow a rated r.p.m., the underspeed valve must be modified sinceotherwise it would always sense an underspeed condition if constructedas shown in FIG. 1, and prevent the transmission from operating belowengine rated speed. A modified underspeed valve is connected across thedifferential pressure stack 74 so that its control spool 111 can open apath parallel to that controlled by the speed control valve 79, aspreviously described. However, the spool is connected to a speed pilotcylinder 112 whose piston 113 is spring-centered in the cylinder. Thispiston has one side in communication.

withthe low-pressure outlet 72a of the venturi 72, and the other side incommunication with the inlet of the venturi through line 114. Thus,changes of flow through the venturi will cause the piston to move from afirst equilibrium position to a newequilibrium position, moving thespool of the underspeed valve through the linkage. Since any equilibriumcondition in the speed pilot cylinder is for a particular speed (flowthrough the venturi), the bias on the cylinder must be changed for anydifferent engine speed selected so that the underspeed valve willfunction properly at the selected engine speed. This change in bias isaccomplished by spring 115 biasingthe spool according to the position ofcam 116 connected by linkage 117 to throttle 118. The throttle also setsthe governor 119 of the engine and through thisarrangement the springbias on the speed pilot cylinder is adjusted so that the underspeedvalve will operate at the speed selected by the throttle rather than ata single preset speed. Through this arrangement the speed controlunitcan be used for selected engine speeds.

Also, in FIG. 3, the actuator and servo systems are shown as simplelinear systems, but the pump pilot cylinder 90 and the motor pilotcylinder 100 are the same and connected to the pressure stack in thesame way as those shown in FIG. 1 except the parallel ball check andorifice units 97 are not shown in the lines of the motor pilot cylinder.

Since the operation of the pilot cylinders have been described, it isonly necessary to describe the operation of the changed actuator system,shown in FIG. 3 for controlling the transmission. Basically, the pilotcylinders are connected through adjustable links 120,. servo spools 121in their respective actuators 122 and 123. Movement of the servo spoolswill open one of the ports 124 communicating between the actuator andthe servo so that pressurized fluid can flow from line 40 to theactuator causing the housing to move in a direction which will close theport repositioning the housing'relative to the servo spool to close theport. The rods 125 of the actuator pistons 126 are secured to the frameand by connecting the housing of the actuator 122 through adjustablelinkage 127 to the pump and likewise connecting the housing of actuator123 through linkage 128 to the motor of the transmission, displacementof the pump and motor can be controlled by the pilot cylinders in themanner previously indicated. The adjustable linkages connecting theactuator housings to the translating units, maybe used to properly nullthe transmission for zero speed.

When operating at higher transmission speeds, where the displacement ofthe.motor has .been reduced to increase speed, the forward-reverse shiftaccomplished by valve 96 may not sequence the transmission properly forthe best torque ratio characteristics, if thernotor pilot cylinder 100is connected as shown in FIG. 1. In FIG. 3, proper sequencing during aforward-reverse shift at high speed is insured by connecting line 101 ofthe motor pilot cylinder to the side of the pump pilot cylinder 90,which is at the higher pressure, through a shuttle unit 129.

The spool 129a of this unit has a center land and two smaller outboardlands with grooves in between, so that when the land is centered, line101 is closed off. When the spool is oifset to one side, line 101 willbe in communication with one of the grooves. One. groove is incommunication with line 95 of the pump pilot cylinder, while the otheris connected with line 94. Thus, depending on the direction of thedisplacement of the spool due to the differential area between thecenter land and the smaller lands at the ends of the spool, one of thepump pilot lines (the line at higher pressure) will be in communicationwith line 101 of the'pump pressure in the motor pilot cylinder. Sincethe lands at opposite ends of the spool are smaller than the centerland, the line operating at the higher pressure will displace the spoolso that it establishes communication with line 101 of the motor pilotcylinder.

Connected in this manner, when a forward-reverse shift is accomplishedby moving valve 96, the higher pressure sides of both the pump and motorpilot cylinders will flow back to the forward and reverse valve, andthence through the restricting ball check and orifice device 97. Sincethe motor pilot cylinder requires a much greater pressure to displaceit, it will first bleed through the system, holding the pump pilotcylinder at its maximum displacement until it has bled down to bottomout. At this time, the pump pilot cylinder will start to bleed down inorder to accomplish a reverse shift. As this has occurred, the higherpressure in the opposite line will begin to reverse the pump pilotcylinder for a reverse power flow, and the spool 129a will shift sofluid pressure will continue to displace the element of the pump pilotcylinder until the pump reaches maximum displacement. In this way, theconnection of the motor pilot cylinder is shuttled between lines 94 and95 of the pump pilot cylinder so that proper sequencing is alwaysinsured during a forward-reverse shift.

The center land will close otf fluid communication of line 101 with theside of the pump pilot cylinder, changing from the lower pressure to thehigher during the forward-reverse shift, until the motor pilot has movedfor maximum displacement and the pump pilot has moved for zerodisplacement, at which time the spool shifts.

Grooves 12917 at each end of the shuttle unit are closed by the smalllands when the spool is in neutral and com municate with the lowpressure side of the stack. Before the spool shifts during aforward-reverse shift, the high pressure from the forward-reverse valve96 will go directly to the low pressure side of the stack until the pumppilot element moves to zero displacement; at this time, the spool shiftsand pressure buildup occurs in both the pump and motor pilots to reversethe transmission. This prevents the higher pressure from working withthe bias to position the element of the pump pilot cylinder to a neutralposition.

FIG. 4 illustrates how FIGS. 5 and 6 are joined together to show acomplete control system for jointly controlling two separatetransmissions 30R and 30L as would be employed in a track-type vehicle.In such a vehicle, steering is effected by driving one track at adifferent speed in relation to the other, and the control systemillustrated provides structures for such a steering capability.

For convenience, the parts of the basic control system and transmissionsin FIGS. 5 and 6 are designated the same numerals where identical asused to describe the embodiment shown in FIG. 1. The heart of thecontrol system shown in FIGS. 5 and 6 is very similar to that shown inFIG. 1, with the exception of the additional structures to'provide thecontrol system with greater flexibility. Specifically, additionalstructures are included to provide for steering in track-type vehicles,braking of the vehicle automatically, correcting overspeed andunderspeed control, override control for control convenience and otherfeatures. complemented by these additional structures, the basic controlsystem becomes a sophisticated system for controlling hydrostatictransmissions in track-type vehicles which is highly efficient andeffective.

Recapping briefly, the differential pressure control system shown inFIGS. 5 and 6 consists of a control pump 70 driven by the engine, aventuri 72, a differential pressure stack 74, a directional valve 96 andpump and motor pilot cylinders and 100, respectively. The above elementsare associated in a manner as previously described and functionsimilarly to the system described in FIG. 1, except insofar as theoperation of the basic system is effected by the additional componentsshown. For convenience, the additional components will be discussedseparately and their purpose, along with their effect on the operationof the basic system, will be noted.

In the differential pressure stack 74, shown in FIG. 5, there is anadditional path between the tubes 75 and 76 controlled by the reliefoverride valve 130. Normally, the spool 131 of this valve is biased tothe closed position by spring 132 so that it has no effect on thedifferential pressure within the stack.

The purpose of the override valve is twofold, in that it provides asafety feature if an overpressure condition develops in thetransmission, and also provides a flexible control for the operator.This latter function is accomplished through plunger 133 in the valvebody which is actuated by operator foot pedal 134 through linkages 135.By means of the pedal, the operator can effect a reduction of thepressure differential within the stack which has been previouslyestablished by setting the speed control lever 82. The spool of thisvalve has metering slots so that depression of the foot pedal willgradually slow the vehicel from the preselected speed range down to stopwithin this range as the opeartor depresses the pedal without using thespeed control lever. When the path through the spool is wide open, thedifferential pressure in the stack will be zero, notwithstanding thesetting speed control valve 82.

Cooperating with the relief and override valve 130 is a safety reliefcylinder 136 which has its plunger 137 spring-biased to a fixed positionwithin the cylinder body. Orientation of the plunger is such that itwill cause the override valve 130 to open if the safety relief cylinderis actuated by high pressure blow-01f from the hydrostatic loop. Thus,if an overpressure occurs within the hydrostatic loop, the saefty reliefcylinder will automatically reduce the differential pressure within thestack, thereby reducing the fluid flow within the hydrostatic loop. Thisprevents the continuation of power flow through the hydrostatic looponce an overpressure has occurred and prevents a large amount of heatfrom being generated in the transmission relief system which couldresult in failure.

In the higher speed ranges of the transmission there is usually notsuflicient compression braking to slow the vehicle under some conditionsand engine overspeed may occur. For this reason it is desirable to havean overspeed control system 139, as shown in FIG. 5. This overspeedcontrol system also uses the differential pressures available in thebasic control system for actuation. Pressure differential developed bythe venturi 72 is used to actuate the vehicle brakes to retard it duringpower generation, such as will occur during vehicle deceleration ordownhill operation. During downhill traverse, for example, powergeneration may occur when the transmission is in the higher speed rangesto the extent that engine compression braking will not be able tocontrol vehicle speed. Under these circumstances, with the motor actingas a pump and the pump as a motor drivingthe engine, an engine overspeedcan occur. The overspeed cylinder 140 uses pressure balancing byconnecting line 141 to the low pressure tap 72a of the venturi andreflecting it on one side of piston 142 in the overspeed cylinder. Onthe opposite side of the piston within the cylinder pressures upstreamof the venturi from line 71 are reflected through line 144 and operateagainst the combined bias of spring 145 and the lower pressure from theventuri tap which normally hold the piston bottomed at one end of thecylinder when no overspeed condition exists. The rod 142a of the pistonis connected through linkages 146 to the air brake boost unit 147 whichcontrols the application of the brakes.

During operation of the overspeed system, brake band 148 on brake drum149 is applied to slow the vehicle during overspeed conditions becausethe pressure in line 141 will decrease due to increased flow through theventuri because of the higher r.p.m. of the control pump 70, andtherefore, the pressure reflected through line 144 will cause the systemto actuate the air control unit for retarding the vehicle until theoverspeed condition is terminated. Other kinds of control units may beemployed.

Another desirable function can be added to the basic control system byincorporating a low speed cut-out valve 155 serially with the underspeedvalve in the path between the tubes 75 and 76 of the differentialpressure stack 74. Basically, the cut-out valve is designed to give thecontrol system two modes of operation by disabling the underspeed valveat low engine r.p.m. so that the vehicle can be moved with the engine ator near idle. Without this cut-out valve, the underspeed valve wouldsense an underspeed condition until the engine was operating near ratedr.p.m. and prevent the transmission from establishing power flow.Incorporation of this cut-out valve allows the transmission to beoperated at or near idle, but without an underspeed control feature.However, in this range underspeed is not a problem.

The cut-out valve is very similar to the underspeed valve but operatesin a reverse mode since it closes the parallel path at lower enginespeed but remains open at higher engine r.p.m. where underspeed controlis necessary. During vehicle operation, low pressure from the lowpressure tap 72a of the venturi is reflected on a floating cut-out spool156 which is biased toward a closed position by spring 157 and thispressure. Reflected on the oppositeside of the cut-out spool throughline 158 is the pressure from the control pump which above certainselected engine speeds will position the spool to open a flow paththrough the valve. However, when engine speed is in the area of idle,the pressures from the low pressure tap are higher and, augmented by thespring bias, will close the cut-out valve. In this condition, the speedcontrol valve can be used to create a differential pressure within thestack since the un derspeed valve is rendered inoperable even though theengine r.p.m. is not at rated.

As noted above, in dual transmissions for track-type vehicles as shownin FIG. 6, means must be provided in the basic control system toindependently control the individual transmissions for steering thevehicle. Control of the motors in dual transmission systems isrelatively easy with the control system of this invention. Basically,the servo control arms 44 of each motor are slaved together throughlinkages and controlled by a single motor pilot cylinder so they reactin unison. In FIG. 6, special scissor linkage is used to slave the servocontrol arms of the motors with one another. This linkage has two bellcranks 151 which are pivoted on appropriate supports and linked to therod 104b of the piston in the motor pilot cylinder so that the bellcranks move together. By this arrangement, the movement of the rod willsimultaneously move the servo control arms of the motors an equal amountso equality of track speed on opposite sides of the vehicle will alwaysbe maintained so long as fluid input to each motor is the same.Adjustable linkages 152 are used to connect the bell crank and the servoarms in the linkage to allow independent adjustment of the motors sothat motor synchronization can be accomplished.

In an embodiment of the control system having a steering capability, asshown in FIG. 6, to control dual transmissions, a single pump pilotcylinder 90 may be employed; and when steering is not being employed,its operation is identical to that described with reference to a singletransmission system. However, to incorporate asteering function, it isnecessary to connect the pump pilot cylinder to the servo control arms41 of the pumps through special linkages so that each transmission canbe independently controlled or operated jointly. Through the steeringlinkage 160 shown complementing the basic control system in FIGS. 5 and6, a greater versatility in steering track-type vehicles is providedsince it allows one track to be reversed in a direction relative to theother track while at all times maintaining the tracks in a poweredcondition.

Specifically, the special steering linkage 160 includes two steeringcylinders 161 each mounted on a pivoted base plate 162. A base plate islocated close to each of the pumps in the dual transmission system, ascan be seen in FIG. 6, and swings about afixed pivot 163. A protrudinglug 164 on each base plate is coupled to a tie rod 165 which spansbetween the transmissions and ties the lugs together so that the baseplates will be swung about their respective pivots in unison. In turn,the tie rod is connected to the pump pilot cylinder 90 which, throughthe tie rod, swings the base plates about their respective pivots inunison. Tie rod 165 includes an adjusting nut 1650 by which therespective base plates may be adjusted relative to one another.

Mounted on each base plate is a steering cylinder Whose piston rod 166is oriented to reciprocate across the axis of the pivot 163 of itsassociated base plate. The travel oft he rod is usually restricted to anequal distance on each side of the pivot axis and each rod connected toa piston 167 within its associated steering cylinder. Each piston inturn is held at one end of the cylinder with a spring 168, and theoutboard end of its associated rod is connected through a linkage 170 tothe servo control arm 41 of the adjacent pump. An adjustment 170a onthis linkage is used to adjust the linkage so the pump has zerodisplacement when the connection between the linkage and the end of therod is directly over the pivot axis of its base plate.

'Once preset to this condition and extended by springs 168, the baseplates are swung about their pivots to change the pump displacement ofboth transmissions an equal amount. Subsequent movement of the rod 166by cylinder actuation to a point directly over the pivot axis willchange the displacement of that pump to Zero and further movement of therod (in the same direction) will carry the connection to the oppositeside of the pivot, which increases the displacement of the pump to itsprior displacement but with a power flow in a reverse direction. Itshould be appreciated as the connection of the rod and linkage moves,the base plate remains stationary so that the transmission on theopposite side of the vehicle remains unchanged. The geometry of thelinkage can be varied to obtain the correct degree of movement for theservo arms.

Using this arrangement, it is possible to set both transmissions at anequal speed for either a forward or reverse direction through controlactuation of the pump pilot cylinder and thereafter by actuation of thesteering cylinder associated with one of the transmissions toindependently slow that transmission and reverse it to the same speedbut in the opposite direction. By increasing the fluid pressure in asteering cylinder sufficiently the requisite movement of the rod can beeffected for such steering operations. The specific linkage described isvery desirable since it operates in either a forward or reversedirection without the necessity of complicated reversing valves or thelike.

The full steering linkage is illustrated in FIG. wherein a bridge valve175 is shown for controlling the pressure flow to the individualsteering cylinders. The bridge valve is really two valves in one whichhas a pair of identical spools joined endwise in the single spool 176.The spool is spring-centered by springs 177 at one end and can beattached to various means for operating it at the other.

The bridge valve has a central port which is connected to a source ofpressure fluid, such as line 40. Fluid pressure entering the valve isrouted to a groove 179 and confined by a land on the spool 176 when itis centered by its centering springs. Movement of the valve to the rightor left first brings one or the other set of small flow orifices 180into communication with the pressure fluid so that it begins to flowfrom the central groove toward one or the other drain grooves 181.Further movement of the spool in the same direction will bring largerslots 182 into communication with central groove so that'the flow willbe increased. As this is occurring another portion of the valve spool issimultaneously closing Off the drain groove with similar passages andslots a and 18211. This causes pressure to build gradually in one of theports 183 between the central groove and one of the drain grooves.'Ihese ports are connected to opposite steering cylinders 161 throughlines 184.

The above bridge system is schematically displayed in FIG. 11, and FIG.12 is a pressure profile developed by the use of the orifice and bleedslots arrangement described above. It can be seen from the pressureprofile 185 that the initial movement of the valve builds the pressureup rather sharply to a steering point and thereafter continued movementof the spool gradually increases the steering pressure up to the pointthat the travel of the rod of one of the steering cylinders begins toreverse the power flow in its associated pump. This is the point wherethe reversal of tracks begins. In this manner, excellent steeringresponse is achieved with a simple, economical system which provides amaximum flexibility. The spool in the bridge valve may be controlled byany suitable means such as a wheel, lever, etc.

A simple steering valve 175 is shown in FIG. 5 and actuated by a lever186 connected to its spool. Lines 184 connecting the valve to thesteering cylinders include a parallel ball check and orifice unit 188which smooth out the steering if desired.

In FIG. 11, the steering circuit just described is schematicallyillustrated. In this particular embodiment, the ports and slots 180,18011, 182 and 182a must provide the pressure profile as shown in FIG.12.

In FIG. 7, dual transmissions in a track-type vehicle 190 areillustrated for a better understanding of the above descriptions.Transmissions 30R and 30L are mounted in the vehicle in a side-by-siderelationship and some of the actuators and other control hardware arealso shown by way of iilustration. The pumps 31 of the transmissions aredriven from a common driveshaft 192 by a bull gear 193 and gears 194.This arrangement allows power regeneration to be interchanged betweenthe transmissions through the common bull gear. Further, no clutch isnecessary since the displacement of the pump in each transmission can beadjusted to zero and power fiow in the transmission stopped. Also, sincethe novel control system is controlled b fluid pressures, the operatorstation can be located remote from the transmission and control system.

Since the transmissions discussed above do not have a clutch betweentheir pumps and the engine, there is a possibility that the vehicle maybe started in gear, i.e., with the speed control lever at go position.Thus, if the operator starts the engine, the vehicle may undergounexpected movement, creating a safety hazard. For this reason, it isdesirable to incorporate safety features in the control system whichprevent unwanted movement on start up. In FIG. 8, an improved relief andreplenishing group 47a is shown in a single housing 200 which providesstart up safety features, as well as the dual functioning elements whichare an improvement over the prior art group shown in FIG. 2.

The replenishing section 201 of the housing 200 includes a cylindricalchamber 202 which has communication with pressurized line 40 and isclosed by poppet spools 203 at opposite ends. Each poppet spool has astepped head 203a arranged so that the stepped surface 203!) hascommunication with an annular recess 204 in the housing which, in turncommunicates through port 205 with one of the conduits in thehydrostatic group, so that one annular recess at one end of the chamberhas communication with conduit 33 while the other recess communicateswith conduit 34.

Springs 206 urge the heads of the poppet spools into chamber 202 wherethey seat to close the chamber. The bias of the springs is such that thepressurized fluid in line 40 will overcome it, depressing the poppetspools and allow fluid into the hydrostatic loop to replenish it withcool hydraulic fluid.

During power fiow in the transmission, one of the conduits will beoperating at a high pressure in order to transmit power from the pump tothe motor, and this higher pressure is reflected via port 205, recess204 and through orifice 207 into a balance chamber 208 behind the poppetspool which will allow pressures to equalize so the poppet spool canmove out of the balance chamber under the influence of the springclosing ofl flow through the replenishing section from the high pressureside of the loop.

In the replenishing section shown in FIG. 8, the balance chambers arevented to reservoir through a channel 208a which is closed by a ballvalve 209. It can be appreciated that unless the ball valve is seated,it will not be possible to operate the transmission since the fluid canbypass the motor through the poppets as described above. The ball valveis seated by directing pressurized fluid via line 213 to plunger 214whose stem portions bear on the ball valves causing them to seat andclose the vent to reservoir. Ball valve 209 may be replaced with othervalve types.

In turn, the safety-reset valve controls the flow of pressurized fluidto the plungers and prevents the transmission from being actuated untilthe operator effects a manipulation in the safety system.

The safety-reset valve 220 is shown in FIG. 9 and when the transmissionis reset pressurizes line 213 through port 221 so that the plungers willclose the ball valves in the replenishing section of housing 200,thereby activating the transmission.

If the park lever (not shown) is released to the off position, link 222will release spool 223 in the housing allowing spring 224 to urge thespool to a position which will open a flow path from pressurized line 40through port 225 into port 226 where the pressurized fluid can pass viaball check 227 to the reset spool 228. The reset spool in turn controlsthe passage of the pressurized fluid to port 221 from which it iscarried to the plungers in the replenishing group for activating thetransmission.

Construction of the reset spool'is such that land 229 closes olf port221 until the spool is moved into spring 230 sufliciently to establishfluid communication between the ball check and this port. When thetransmission is disabled, the reset spool will have the position shownin FIG. 9 so that pressurized fluid passing the ball check can, throughorifice 231, pass into chamber 232 by land 233. A bleed port 234 in thespool bleeds chamber 232 to reservoir when the spool is in the safetyposition, as can be seen in the drawing. In operation, after the parklever is released and spool 223 opens a flow path of pressurized fluidto ball check 227, the reset spool can be pushed into its spring whichwill cause passage 234 to be closed by the valve body and allow fluidpressure passing the ball check and acting on land 233 to hold the resetspool against the spring bias. In this positon, it allows thepressurized fluid to pass into the port 221. The reset spool isgenerally located adjacent to the speed control lever and oriented sothat when the speed control lever is returned to the stop position, itwill depress the spool which will be then held by fluid pressure in theon position, providing the park lever is released.

To prevent hydraulically holding the reset spool in the activatedposition once set through a hydraulic lock, chamber 232 is bled throughport 235 via metering valve 236 to reservoir, which will allow the fluidtrapped in chamber 232 to bleed slowly to reservoir, thereby allowingthe reset spool to move under the spring pressure to open the chamber toreservoir and disable the transmission when fiiud flow to the spoolfalls below the quantity being bled from the chamber. This wouldnormally occur on shut down or placement of the park lever in the onposition. In this way, the transmission is disabled until the park leveris released and the speed control lever is returned to stop. Thisprovides a very desirable safety feature for hydrostatic transmissionsand this safety feature will eliminate some of the hazards that will bepresent due to the operators unfamiliarity with a hydrostatic type oftransmission system in track-type vehicles.

The vent and overpressure relief functions are also combined in thehousing 200 shown in FIG. 8 in the event overpressure section 240. Thissection has a master chamber 241 which is divided into two secondarychambers by a shuttle piston 242 which is free to reciprocate betweenthe chambers, but prevents fluid communication therebetween. One of thesecondary chambers 241a is connected through its port 243 to conduit 33and the other secondary chamber through its port 233 is connected toconduit 34. Located at each end of the master chamber chamber are poppetvalves 246 which close ports 247 at opposite ends of the master chambertherefrom. These ports lead via passage 248 to reservoir. Each poppetvalve is biased by a spring 249 to close the communication between itsassociated port and secondary chamber. Behind each poppet valve is abalance chamber 250 which drains to reservoir via port 251, which inturn is closed by a spring-loaded poppet 252. Each poppet valve has anorifice 253 in its face so that the pressre in the secondary chamberscan pass through the orifice to the balance chamber to equalizepressures.

Utilizing the above construction, when the transmission is activated,the vent and overpressure valve will function in the following manner.The pressure from either conduit 33 or 34, the one operated at thehigher pressure, will enter one of the secondary chambers 241a throughport 243 and cause the shuttle spool to move into the opposite secondarychamber. As this occurs, a nose piece 242a on the end of the shuttlepiston will engage the face of the poppet valve 246 depressing it andopen the lower pressure'conduit through ports 243 and 247 and ivacoolers and relief valves to reservoir so that additional fluid will beremoved from the low pressure side of the hydrostatic loop so additionalfluid can be added for cooling. The opposite poppet then becomes thehigh pressure relief overpressure valve since high pressure fluid in itssecondary chamber may enter the balance chamber behind its poppet valveand, with the assistance of the spring, hold this poppet valve closed aslong as the pressure in the balance chamber does not exceed the springbias on the small poppet 252. Should the latter occur, high pressurefluid will be vented from the balance chamber into ports 254 to the highpressure override cylinder 126 (see FIG. 5) via line 255 and the poppetvalve will dump to the opposite conduit (33 or 34) through passage 248.

During the above description of the basic control system, mention wasmade of parallel ball check and orifice units 97 which were used tosmooth out the operation of the control system so that abrupt changes insetting would not be accompanied with violent transmission response.While the parallel ball and orifice units do smooth the transmissionresponse, the rate of response varies in proportion to the differentialpressure established in the stack 74. For this reason, it is desirableto use a variable orifice device 270, shown in FIG. 13. The variableorifice device includes a housing 271 having a bore 272 therein.Reciprocably mounted within this bore is a spool 273 that has a land atboth ends and a reduced diameter center portion. In the area of reducedcenter portion, a plurality of radial orifices 274 is drilled ingenerally axial alignment so that as the spool moves within the boreagainst the bias of spring 275, one of its lands 273a will seriallyclose the orifices and thereby restrict fluid flow from the bore tochamber 276. Fluid is supplied to the bore between the lands of thespool via an axial passage 277 in the spool which extends from one endthereof and vents into bore 272 between the lands. By this arrangement,fluid pressure entering the unit via port 278 can pass through the axialpassages and thence through the orifices to chamber 276-. Spring 275biasing the spool 273 to keep the orifices open is located in a balancechamber whose port 279 is in communication with tube 76 of theditferential pressure stack.

Utilizing this unit, it is possible to give the control sys- 17 tem asmooth response over its full range independent of the differentialpressure developed in the stack.

By way of illustration, reference is made to FIG. 1 wherein the ballcheck and parallel orifice device 97 may be disconnected from line 101and this line in turn connected to chamber 276 with port 278 connecteddirectly to tube 75 of the differential pressure stack. Connected inthis manner, the greater the pressure differential in port 278 to thatin port 279, the more the spool will move into spring 275, closing offmore and more of the orifices 274. Thus, at higher differentialpressures, fewer orifices will be available for the transmission ofhydraulic fluid to the pump pilot cylinder 90. Conversely at lowerdifferential pressures more of the orifice passages will be exposed andrepsonse will tend to be uniform throughout the control range. As thedifferential pressure is reduced and reverse flow through the unitoccurs, spring 275 will displace the spool so that all the orifices areopen from chamber 276 to port 278, so that in effect it operates likethe check valve.

Utilizing these differential orifice devices which, like many of thecomponents of the control system, are based on differential pressureprinciples, it is possible to give the basic control system smoothrepsonse troughout its entire control range so that operator comfort andsafety will be maintained. Also, it should be appreciated when thevariable orifice units are connected in the control system, port 278will be in communication with either the high pressure tube 75 of thedifferential pressure stack, or tubes 95 and 102 of the pump pilotcylinder and motor pilot cylinder, respectively. This can be appreciatedby noting the orientation of the ball and check orifice device 97 inFIG. 1 for which variable orifice units 270 are substituted in therefined version of the control system or other means.

An alternative embodiment of the differential orifice device can befabricated by locating port 276 so that it has direct communication withbore 272 in the area of the reduced portion of the spool 273- anddeleting the axial passage 277 in the spool. In this embodiment one endof the spool is connected with one side of the pilot cylinder and theopposite end of the spool is connected with the other side of the pilotcylinder being modulated. Connected in this manner the unit providesadditional modulation since the fluid passing to or from the pilotcylinder through port 278 is modulated in proportion to the pressures onthe opposite sides of the pilot cylinder.

In the drawings, R is used to refer to a common reservoir.

What is claimed is:

1. A semi-automatic control system for engine driven hydrostatictransmissions with a variable displacement pump which is capable ofautomatically adjusting the transmission in relation to input availablewith torque output requirements within selected ranges:

(a) a positive displacement control pump geared to theengine-transmission drive system having a fluid output directlypropotrional to engine speed;

(b) a venturi means having an inlet, an outlet, and a low pressure tap,said venturi means having its inlet connected to receive the fluidoutput of said control P p;

(c) differential pressure generating means having a first chamber meansconnected to receive said fluid output of said control pump, a secondchamber means connected to said first chamber with passage means, saidpassage means including fluid restricting means whereby a variablepressure differential between said first chamber means and said secondchamber means can be established by adjusting said fluid restrictingmeans during flow of said fluid output through said differentialpressure generating means;

(d) venturi responsive control means connected to said restrictingmeans, said control means connected to sense pressure in said inlet andsaid low pressure tap of said venturi means and thereby operable to varythe adjustment of said restricting means as a function of engine speed;(e) differential pressure responsive means connected ,via a firstconduit means to receive a fluid pressure signal from said first chambermeans and connected via a second conduit means to receive a fluidpressure signal from said second chamber means of said pressuredifferential generating means, said differential pressure responsivemeans having a mechanical output proportional to the differentialpressure hetween said chambers; and

(f) a servo system connected to said mechanical output of saiddifferential pressure responsive means and associated with said variabledisplacement pump of said hydrostatic transmission whereby thetransmission ratio is controlled as a function of said differentialpressure in the control system which is ina dependent of the pressure inthe hydrostatic loop of the transmission.

2. A semi-automatic control system as defined in claim 1 wherein thepositive displacement control pump has its output connected to the inletof the venturi means and the, outlet of said venturi means is connectedto the first chamber means in the differential pressure generatingmeans.

3. A semi-automatic control system as defined in claim 1 wherein thepositive displacement control pump has its output connected to the firstchamber means of the differential pressure generating means and thesecond chamber, means of said differential pressure generating means isgonnected to the inlet of the venturi means.

4. A semi-automatic control system as defined in claim 1 wherein thepassage means between the first chamber means and the second chambermeans of the differential pressure generating means includes at leasttwo separate passages and the fluid restricting means includes amanually positioned restricting valve means for controlling flow throughone of said passages and a speed control valve means controlling flowthrough the other of said passages, said speed control valve meansoperably connected to the venturi responsive control means whereby thepressure differential between the first chamber means and the secondchamber means of said differential pressure generating means is afunction of both the setting of the manually positioned restrictingvalve means and engine speed.

5. A semi-automatic control system as defined in claim 4 wherein theventuri responsive control means include an adjustable biasing meanswhereby the speed control valve means can be properly biased fordifferent control pump and engine speeds.

6. A semi-automatic control system as defined in claim 4 wherein anindependent bypass passage and valve means is included between the firstchamber means and the second chamber means of the differential pressuregenerating means and actuating means associated therewith which isresponsive to a signal from the operator or a' signal from anoverpressure sensor in the power loop of the hydrostatic transmissionwhereby such signals will reduce the differential pressure between saidfirst chamber means and said second chamber means when said bypasspassage and valve means are actuated, notwithstanding the setting of thefluid restrictive means.

7. A semi-automatic control system as defined in claim 4 wherein thespeed control valve means includes cutoff valve means isolating it fromits control function at speeds below preselected engine speed so thatnominal transmission ratios will be available without an engine speedcontrol function parameter in the control system.

8. A semi-automatic control system as defined in claim 1 wherein thepressure responsive means include fluid actuated means connected to thefirst chamber means through a first conduit and the second chamber meansthrough a second conduit of the differential pressure generating means,said fluid actuated means including an element therein the position ofwhich is proportional to the differential pressure between said firstchamber means and said second chamber means for producing the mechanicaloutput.

9. A semi-automatic control system as defined in claim 1 wherein thesystem includes braking cylinder control means operably associated witha braking system in the transmission drive train, said braking cylindercontrol means connected to the inlet of the venturi means and its lowpressure tap whereby engine speeds in excess of those preselected in thecontrol system will cause said braking cylinder control means to applysaid brakes to protect said engine from over speeds.

10. A semi-automatic control system as defined in claim 8 wherein amanually operated switching valve means is connected in the firstconduit means and the second conduit means connecting the first chambermeans and the second chamber means with the differential pressureresponsive means whereby said conduits will be reversed in theirconnections with said differential pressure responsive means in order toachieve equal but opposite mechanical outputs for reverse power flowthrough the transmission.

11. A semi-automatic control system as defined in claim 8 wherein thefirst conduit means and the second conduit means which connect the firstchamber means and the second chamber means of the differential pressuregenerating means with the differential pressure responsive means eachinclude a flow control valve means with variable orifice means tomaintain a constant fiow capacity independent of pressure level toachieve uniform response in changes of transmission ratio at all speeds.

12. A semi-automatic control system as defined in claim 1 for enginedriven transmissions having both a variable displacement pump and avariable displacement motor wherein a second differential pressureresponsive means which is connected to the first chamber means by athird conduit means of the differential pressure generating means andwhich is connected to the second chamber means by a fourth conduit meansof said differential pressure generating means, said second differentialpressure responsive means including pressure sensitive means preventinga mechanical output until the differential pressure responsive means hasachieved maximum output, and a motor servo system connected to themechanical output of said second differential pressure responsive meanswhereby the transmission speed ratio is increased by decreasing thedisplacement of its variable displacement motor subsequent to the timeits variable displacement pump unit has reached maximum displacement,thereby achieving proper sequencing of the transmission.

13. A semi-automatic control system as defined in claim for enginedriven transmissions having both a variable displacement pump and avariable displacement motor wherein a second differential pressureresponsive means which is connected to the first chamber means by athird conduit means of the differential pressure generating means andwhich is connected to the second chamber means by a fourth conduit meansof said differential pressure generating means, said second differentialpressure responsive means including pressure sensitive means preventinga mechanical output until the differential pressure responsive means hasachieved maximum mechanical output, and a motor servo system connectedto the mechanical output of said second differential pressure responsivemeans whereby the transmission speed ratio is increased by decreasingthe displacement of its variable displacement motor subsequent to thetime its variable displacement pump has reached maximum displacement,thereby achieveing proper sequencing of the transmission.

14. A semi-automatic control system as defined in claim 10 for enginedriven transmissions having both a variable displacement pump and avariable displacement motor wherein a second differential pressureresponsive means which is connected to the first chamber means by athird conduit means of the differential pressure generating means andwhich is connected to the second chamber means by a fourth conduit meansof said differential pressure generating means, said second differentialpressure responsive means including pressure sensitive means preventinga mechanical output until the differential pressure responsive means hasachieved maximum output, and a motor servo system connected to themechanical output of said second differential pressure responsive meanswhereby the transmission speed ratio is increased by decreasing thedisplacement of its variable displacement motor subsequent to the timeits variable displacement pump has reached maximum displacement, therebyachieving proper sequencing of the transmission.

15. A semi-automatic control system as defined in claim 12 wherein thepressure sensitive means include shuttle valve means to preventdifferential pressures from reaching the second differential responsivemeans until the differential pressure responsive means associated withthe pump has reached its maximum mechanical output.

16. A semi-automatic control system as defined in claim 12 wherein thedifferential pressure responsive means includes a cylinder having apiston centrally disposed therein with biasing means to centrally locatesaid piston when fluid pressures on opposite sides thereof are equal andthe second differential pressure responsive means includes a cylinderhaving a piston biased to one end thereof with spring means so it willrespond to pressures only exceeding those effecting a maximum mechanicaloutput of said differential pressure responsive means.

17. A semi-automatic control system as defined in claim 6 wherein thepower loop of the hydrostatic transmission includes a high pressurerelief valve and means connecting it to the bypass passage and valvemeans in the differential pressure generating means whereby an overpressure in said power loop will operate said bypass passage and valvemeans to reduce the differential pressure and prevent overheating insaid hydrostatic transmission.

18. A semi-automatic control system as defined in claim 17 wherein highpressure fluid from the transmission power loop passing the highpressure relief valve operates the bypass passage and valve means.

References Cited UNITED STATES PATENTS 2,562,615 7/1951 Huber 103-372,659,425 11/ 1953 Ifield. 3,284,999 11/1966 Lease. 3,302,389 2/1967Cadiou. 3,302,487 2/ 1967 Kempson. 3,230,699 1/ 1966 Hann et a1.

EDGAR W. GEOGHEGAN, Primary Examiner US. Cl. X.R. -52, 53

